The original maize plant was indigenous to the Western Hemisphere. The plants were weedlike and only through the efforts of early breeders was a cultivated crop species developed. The physical traits of maize are such that self pollination or cross pollination can occur. Each plant has a separate male and female flower, the tassel and ear, respectively. Natural pollination occurs when wind transfers pollen from tassel to the silks on the corn ears. This type of pollination contributed to the wide variation of maize varieties present in the Western Hemisphere.
The development of a planned breeding program for maize only occurred in the last century. Originally, maize was an open pollinated variety having heterogeneous genotypes. The maize farmer selected uniform ears from the yield of these genotypes and reserved them for planting the next season. The result was a field of maize plants that are segregating for a variety of traits. This type of maize selection lead to at most incremental increases in seed yield.
Large increases in seed yield were the result of the development of hybrid corn varieties in planned breeding programs. Hybrids were developed by selecting corn lines and selfing these lines for several generations to develop homozygous pure inbred lines and crossing selected inbred lines with unrelated inbred lines to produce hybrid progeny (F1). Inbred lines can be difficult to produce since the inbreeding process in corn decreases the vigor. However, when two inbred lines are crossed, the hybrid plant evidences greatly increased vigor compared to open pollinated segregating maize plants. An important factor of the homozygosity and the homogeneity of the inbred lines is that the hybrid from any cross will always be the same, and can be reproduced.
The ultimate objective of the commercial maize seed companies is to produce high yielding, agronomically sound plants which perform well in certain regions or areas of the Corn Belt. To produce these types of hybrids, the companies must develop inbreds which carry needed traits into the hybrid combination. Hybrids are not uniformly adapted for the Corn Belt, but are specifically adapted for regions of the Corn Belt. Northern regions of the Corn Belt require shorter season hybrids than do southern regions of the Corn Belt. Hybrids that grow well in Colorado and Nebraska soils may not flourish in rich Illinois soil. Thus, a variety of major agronomic traits are important in hybrid combination for the various Corn Belt regions, and have an impact on hybrid performance.
Inbred line development and hybrid testing have been emphasized in the past half century in commercial maize production as a means to increase hybrid performance. Inbred development is usually done by pedigree selection. Pedigree selection can be selection in an F2 population produced from a planned cross of two genotypes (often elite inbred lines), or selection of progeny of synthetic varieties, open pollinated, composite, or backcross populations. This type of selection is effective for highly inheritable traits, but other traits, for example, yield requires replicated test. crosses at a variety of stages for accurate selection.
Maize breeders select for a variety of traits in inbreds that impact hybrid performance along with selecting for acceptable parental traits. Such traits include yield potential in hybrid combination; dry down; maturity; grain moisture at harvest; greensnap; resistance to root lodging; resistance to stalk lodging; grain quality; disease and insect resistance; ear and plant height; performance in different soil types such as: low level of organic matter, clay, sand, black, high pH, low pH; performance in: wet environments, drought environments, and no tillage conditions. These traits appear to be governed by a complex genetic system that makes selection and breeding of an inbred line extremely difficult. Even if an inbred in hybrid combination has excellent yield (a desired characteristic), it may not be useful because it fails to have acceptable parental traits such as seed yield, seed size, pollen production, good silks, plant height, etc.
To illustrate the difficulty of breeding and developing inbred lines, the following example is given. Two inbreds compared for similarity of 29 traits differed significantly for 18 traits between the two lines. If 18 simply inherited single gene traits were polymorphic with gene frequencies of 0.5 in the parental lines, and assuming independent segregation (as would essentially be the case if each trait resided on a different chromosome arm), then the specific combination of these traits as embodied in an inbred would only be expected to become fixed at a rate of one in 262,144 possible homozygous genetic combinations. Selection of the specific inbred combination is also influenced by the specific selection environment on many of these 18 traits which makes the probability of obtaining this one inbred even more remote. Thus, the general procedure of producing a non segregating F.sub.1 generation and self pollinating to produce a F.sub.2 generation that segregates for traits does not easily lead to a useful inbred. Great care and breeder expertise must be used in selection of breeding material to continue to increase yield and agronomics of inbreds and resultant commercial hybrids.